As is known from DE 10 2006 033 567 A1, particle filters are activated in the exhaust gas system of diesel engines to reduce the particle emission, in particular a soot emission. When operating a diesel engine, particles collect on the filter surface of the particle filter or in the filter medium thereof. Eliminating the soot by means of a soot burn-off and therefore by oxidation is known method for regenerating such a particle filter. This type of soot burn-off occurs automatically when the exhaust gas temperature flowing against the particle filter is greater than the ignition temperature of the soot. Fuel additives can be used to reduce the soot ignition temperature. Thus, an ignition can also take place by adding these types of additives at a temperature below the soot ignition temperature, but above the soot ignition temperature with additives. The exhaust gas temperature then normally exceeds the soot ignition temperature or said soot ignition temperature with the addition of additives if the diesel engine is operating under a specific load over a specific time period. Active regeneration processes are used in order to make a regeneration of the particle filter possible also in operating states in which the exhaust gas temperature is lower than the soot ignition temperature also with the addition of additives. This is accomplished by supplying thermal energy, for instance via thermoelectric heating elements or by injecting fuel into the exhaust gas stream.
To make it possible for a discontinuous regeneration of the particle filter to occur through soot burn-off, it is necessary for the particle filter to have a specific soot load or to not exceed said load. If the soot quantity accumulated on the particle filter is too low, this can produce an incomplete, uneven soot burn-off, and if the soot quantity is too great, uncontrolled combustion processes and therefore damage may occur. Therefore, the point in time at which regeneration is started is essential for the regulation of a particle filter system. If this takes place too soon, e.g., too frequently, there is not enough material (soot) on the filter for regeneration to proceed completely, in other words, for the entire filter to self-clean. For the filter elements used, there are, e.g., sintered metal filters or even ceramic filter elements, e.g., based on silicon carbide, cordierite or aluminum titanate, and an upper and lower limit for the soot load between which a regeneration takes place optimally. As a result, the soot load must be observed and as soon as the application is in the optimal “soot window”, a suitable point in time must be determined for igniting the regeneration.
The correct additive to diesel fuel ratio is also important for the success of a regeneration with a diesel particulate filter. In the prior art, the fuel content in the tank is frequently measured via the tank sensor. The quantity of additive required is calculated with the measured fuel quantity and added to the fuel tank using a metering pump.
Patent EP 1 509 691 B1 describes the start of a regeneration as a function of the soot load on the filter. The pressure difference in the exhaust gas after-treatment system before and after the particle filter is determined as an essential parameter. In addition, a theoretic particle load is calculated and the load state of the particle filter is determined by comparing the theoretic particle load with the particle load measured from the pressure difference, wherein to calculate the theoretic particle load, the exhaust gas temperature minus an assessment threshold is integrated over a predetermined time. An additive is added to the fuel, when the load state of the particle filter has exceeded a specific threshold. The quantity of added additive can come about as a function of the concentration of the additive in the tank.
Patent EP 1 583 892 B1 describes a method for regulating the actuation of a heating apparatus for regenerating a particle filter activated in the exhaust gas system of an internal combustion engine. In the case of this method, the exhaust gas backpressure generated by the particle filter is detected as a measure of the soot load state of the particle filter and the exhaust gas backpressure is measured when the engine is idling and the exhaust gas recirculation is switched off. Then the exhaust gas backpressure signal is compared with a threshold value representing an adequate soot load for triggering a regeneration of the particle filter. The heating apparatus for triggering the regeneration process is enabled when the detected exhaust gas backpressure signal is greater than the threshold value. The disadvantage here is that the exhaust gas backpressure cannot be measured during ongoing normal operation, but only when idling.
A minimum of a two-stage method for determining the starting time of the regeneration process of a particle filter activated in the exhaust gas system of an internal combustion engine is known from patent DE 10 2006 033 567 A1 that was already cited in the foregoing. In this case, first the current soot load state of the particle filter is determined. Said soot load state is compared with a characteristic map constructed of data which represent the soot load required for a regeneration process with adequate regeneration success in the case of different operating states of the internal combustion engine. If the currently determined soot load is greater than or equal to the minimum soot load required by the characteristic map, a “load state OK” flag is set. Once the “load state OK” flag is set, the expected regeneration success for it is determined, if the regeneration process would be triggered at the point in time of the determination as a function of the current operating state of the internal combustion engine. If this regeneration success is adequate, a “start regeneration” flag is set.
A method is known from patent application EP 2 252 780 for determining the load state of a particle filter activated in the exhaust gas system of an internal combustion engine, in particular of a diesel engine, which comprises the following steps: Determining the exhaust gas volume flow in the flow direction of the exhaust gas after the particle filter; Detecting the prevailing pressure in the exhaust gas system in the flow direction of the exhaust gas before the particle filter; Comparing the exhaust gas volume flow determined in the flow direction after the particle filter with the prevailing pressure determined before the particle filter; and evaluating the result of the comparison taking into consideration the exhaust gas backpressure made available by the unloaded particle filter with respect to exhaust gas backpressure producing the particle filter load.
EP 1 736 653 describes the metering of the additive for regenerating a diesel particulate filter as a function of the operating conditions of a motor vehicle.
The addition of an additive for regenerating a diesel particulate filter as a function of the engine load is known from EP 0 488 831.